Gamma→Alpha Linkage and Persistent Firing of Ia Fibers by Pudendal Nerve Stimulation in the Decerebrate Cat

Abstract

The sensory pudendal nerve (SPN) was stimulated in decerebrate female cats. Spikes of single Ia muscle spindle afferents from the medial gastrocnemius (MG) muscle were recorded in dorsal root filaments. Electroneurography (ENG) was recorded in a cut nerve filament to the MG muscle; MG electromyography (EMG) was also recorded. Single shock to SPN induced discharges of small ENG spikes (SS) with similar amplitude to that of gamma spikes elicited by ventral root stimulation. Thus SS were identified as gamma spikes. The latency of the gamma discharge was ∼15 ms. As expected, the onset of the gamma discharge preceded a discharge of Ia spikes; the time difference between both discharges was ∼5 ms. After the initial bursts, the Ia and the gamma activities paused during 20–30 ms but later increased again to last ∼1 s. After the shock, the EMG activity was depressed during ∼50 ms; later, motor-unit spikes may show transient activation. Thus the onset of the gamma activation preceded the activation of motor units (gamma→alpha link). Trains of shocks (1 or 100 Hz) to SPN induced a sustained increase in the frequency of gamma spikes, Ia spikes, and motor units that outlasted the train by 20–120 s. The sustained firing of Ia fibers might trigger or help to trigger and maintain the response of alpha-motoneurons.